Climatic role

• Is a main transporter of heat from the tropics to northern latitudes
• Path of ocean currents affects agriculture and habitability of regions, for example:

 

	~61o N	~62o N
	Faeroe Islands	Anchorage Alaska
Mean January temp	40o F	13o F

• Driven by winds, not thermohaline processes
• In the upper few hundred metres of the ocean
• Clockwise gyres in North Atlantic and North Pacific
• Anti-clockwise gyres in South Atlantic and South Pacific
• Gyres are underneath, and driven by, the bands of Trade Winds and Westerlies
• Controlling forces:
• Wind friction on the surface of the ocean
• Coriolis Force
• Gravity
• Shapes of the ocean basins

• Several currents:
• North Atlantic Current driven by westerlies
• North Equatorial Current driven by trade winds
• Currents turn when "blocked" by continents
• Wind driven currents are moving to the right of the prevailing wind because of Coriolis force

• Similar current structure
• Winds directions are named for where they come from, currents directions are named for where they go to

1. Consider the surface ocean as a series of thin layers
2. Wind friction pushes the surface layer in wind direction
3. Coriolis force turns this motion to the right
4. Next lower layer is pushed by the layer above, its movement is also deflected to the right
5. Continues through many layers, each turning to the right of the layer above
6. Only transmit small amount of energy to each lower layer so push on layers gets weaker with depth
7. Flow in bottom layer is in opposite direction to surface, but very weak
8. Called the Ekman Spiral, total layer down to reverse in flow direction is called the Ekman layer
9. Net motion in the Ekman Layer is 90^o to the right of the wind in the northern hemisphere
10. Is 90^o to the left of the wind in the southern hemisphere
11. The transport of water in this layer is called Ekman transport

• Ekman transport piles up water toward the centre of the gyre
• Force of gravity on the piled up water creates pressure gradient away from centre of the gyre
• The balance between these forces is known as geostrophic balance
• Results in the transport in the Ekman layer being about 45^o to the right of the wind instead of 90^o
• The flow in the gyres is controlled by geostrophic balance and is called geostrophic flow, the balance of wind energy and gravity
• Geostrophic "hill" is closer to the western side of the gyre because of Earth's rotation

Currents

• Geostrophic balance keeps currents on boundary of gyre
• Trade wind belts converge at Equator, pile up water on western side of oceans

• Currents on western side of the ocean (western boundary currents) are narrow and fast flowing
• Currents on the eastern side of the basin are slower and more diffuse
• Western boundary currents are warm as they come from the Equator
• Eastern boundary currents are cold